Wondering How Old That Stocked Fish Is? Take A Look At The Fins!

By IFW Fisheries Biologist Kevin Gallant

Springtime is when fish culturists at the state’s hatcheries are busy preparing to stock lakes and rivers for anglers. Many of the fish we stock have had a fin clipped. IF&W fisheries biologists, fish culturists, and even volunteers have been “marking” fish over the past several weeks by removing a fin or combination of fins in order to identify and age these fish later.

Why do we mark fish by clipping fins? Fisheries biologists use fin marks to identify a fish’s year of stocking. Knowing the “mark of the year”, we can easily determine how old a fish is when caught or captured in one of our lake surveys. Marks used in certain waterbodies can determine age and growth rates from year to year, or provide an indication of how many fish survive from season to season. To view a short video, check this out: https://youtu.be/P0l6b0zls2U.

In turn, growth rates, or health, of fish can be used to determine such biological factors as forage availability and competition in each waterbody. We can then use this information to adjust future stocking rates for that particular water.

There are several different techniques used to mark fish, however, fin clipping is a simple method that is inexpensive, quick, and has the advantage of being easily seen. Not all hatchery fish get marked; however more than 117,000 fish were given a fin clip this spring so at the time of harvest or capture they can be identified as hatchery origin.

IF&W fisheries biologists use a four-year rotation (“mark of the year”) of fin clips for short-lived salmonids such as landlocked salmon and brook trout. For the four-year rotation, the only fins used for marking are the ventral fins (also known as pelvic fins) and the adipose fin. For instance, one year the mark will be the left ventral fin (E), the following year will be both ventral fins (E+F), the year after it would be the right ventral fin (F), and finally the fourth year would be an adipose fin (B) clip. An eleven-year fin clip rotation is used for the longer-lived lake trout with the pectoral fins (C+D) also being used to increase the amount of combinations with the ventral and adipose fins. It’s estimated an experienced clipper can mark between 400-800 fish per hour, depending on the mark. The approximate labor cost of fish marking is anywhere between $0.05-$0.09 per fish, which varies depending on fish size and mark used.

Not only do fin clips provide biologists with important information regarding the age of a fish, anglers can easily identify these clips as well. So, when you’re out fishing this season keep an eye open for fin clips and you can tell how old your catch is.

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